This invention relates generally to a system for encoding binary data words into a converted digital code for transmission and, more particularly, is directed to a system for encoding binary data words into a converted digital code for transmission and having a format with substantially no components thereof in a predetermined undesirable frequency range.
In digital transmission systems and in magnetic and optical recording/reproducing systems, the information to be transmitted or to be recorded is presented as a sequence of symbols. These symbols collectively form an alphabet, which for binary data is a binary alphabet consisting of the symbols "1" and "0". One symbol, for example, the "1", can be recorded in accordance with a non-return-to-zero-mark (NRZ-M) code as a transition between two states of magnetization in the case of a magnetic disc or tape, or between two states of intensity or focus in the case of an optical disc. The other symbol, the "0", is recorded by the absence of such a transition. As discussed in greater detail below, certain system requirements in practice impose constraints on the order in which these symbols may occur.
Some systems are required to be self-clocking. This implies that the sequence of symbols to be transmitted or to be recorded should have sufficient transitions, that is, should be of a so-called run length limited (RLL) code, so that a clock signal, which is required for detection and synchronization, can be generated and regulated by the sequence of symbols alone. Accordingly, a predetermined maximum distance between two consecutive transitions on the recording medium, and consequently, a maximum time interval (T.sub.max) corresponding therewith, of the signal to be recorded, must be provided. A further requirement may be that certain symbol sequences are forbidden in the information signal, as these sequences are intended for special purposes, for example as a synchronizing sequence. If the synchronizing sequence is mimicked by a sequence in the information signal, the information signal destroys the unambiguity of the synchronizing sequence, and consequently, renders the synchronizing sequence unsuitable for its intended purpose.
It may further be required that the transitions not follow too closely after each other in order to limit intersymbol interference. In the case of magnetic or optical recording, the latter requirement may also be related to the information density on the recording medium. For example, if the predetermined minimum distance between two consecutive transitions on the recording medium, and consequently, the minimum time interval (T.sub.min) corresponding therewith, of the signal to be recorded is increased, the information density is increased substantially to the same extent, that is, with the same ratio. Also, the required minimum bandwidth (B.sub.min) relates to the minimum time interval T.sub.min between transitions, that is, B.sub.min =1/2 T.sub.min.
A method of encoding data to satisfy the above requirements is disclosed in U.S. patent application Ser. No. 06/286,982, filed July 27, 1981, and having a common assignee herewith. In particular, as described therein, the encoded binary digital signal must satisfy a d-constraint rule and a k-constraint rule, as follows:
(a) d-constraint rule - two "1"-type symbols must be separated by a run of at least d consecutive symbols of the "0" type; and
(b) k-constraint rule - the maximum length of a run of consecutive symbols of the "0" type is k.
For example, if a stream of binary data is considered as consecutive sequential words each having m data bits, the words can be encoded into corresponding code words of n information bits, where n&gt;m. Since n&gt;m, the number of possible code words (2.sup.n) far exceeds the number of different data words (2.sup.m), and only those code words need be used which satisfy the d-constraint rule and the k-constraint rule. In other words, if the d- and k-constraint rules are imposed on the code words of n information bits to be transmitted or recorded, the 2.sup.m different words map onto a corresponding 2.sup.m words of information bits out of a total possible number of 2.sup.n such words. Thus, the mapping of data words into code words is carried out so that only those code words are used that satisfy the k-constraint and d-constraint rules.
In addition, it is desirable to maintain the DC component of the sequence of symbols as close to zero as possible, because information channels are not normally responsive to direct current and any DC component of the transmitted or recorded signal is likely to be lost. It has therefore been proposed in the aforementioned U.S. patent application Ser. No. 06/286,982, to add a redundant or separation word, for example, of three bits, to each encoded binary digital word to form a new channel word with substantially no DC components, that is, in which DC imbalance is substantially reduced, while also satisfying the d-constraint and k-constraint rules. For example, with such method, an m-bit digital word is mapped onto an n.sub.1 -bit digital word which satisfies the d-constraint and k-constraint rules and in which a transition between a high and low state is provided for each occurrence of a "1" bit in each n.sub.1 -bit word. The instantaneous value of the integration of an encoded signal is called the digital sum value (DSV), and includes information about the DC content of the signal. More particularly, the closer that the DSV is to zero, the fewer DC components exist in the signal. In this manner, an n.sub.2 -bit redundant word can be added to each n.sub.1 -bit word to form a new n-bit channel word in which the DSV is as close to zero as possible, while also satisfying the d-constraint and k-constraint rules.
The above system is only directed to reducing DC imbalance, while satisfying the d-constraint and k-constraint rules. However, it may also be desirable to provide that the encoded binary digital signal has substantially no components in a predetermined undesirable frequency range. For example, with an optical disc reproducing apparatus, a binary digital information signal is recorded on an optical disk by the presence or absence of pits thereon. In particular, with such apparatus, an optical laser beam may be reflected from the surface of the recording medium to detect the presence or absence of pits and the spacing between adjacent pits, whereby the reflected laser beam is modulated by the binary digital information on the recording medium. The reflected laser beam is then detected to produce a detection or information signal containing the recorded information. However, if a stain or flaw, such as a fingerprint, exists on the disc, the detection signal may become distorted by an asymmetrical AM component thereof, and reproduction with high fidelity is therefore not achieved. Since the added component from the fingerprint appears as a low frequency component, it is desirable that the reproduced binary digital information signal has a format with substantially no components in such low frequency spectrum caused by the fingerprint, whereby reproduction with high fidelity can be achieved.
Another problem that may result with such optical disc reproducing apparatus relates to the tracking of the pits within the tracks on the disc. In particular, tracking by a servo mirror and the depth of focus of a servo lens of the apparatus are controlled on the basis of the envelope of the reproduced detection signal. However, low frequency components of the detection signal itself may cause a drift in the envelope thereof, resulting in inaccurate tracking and focusing of the laser beam. In addition, as a result of a phase compensation operation, the tracking loop has a saturation phenomenon which results in a track jump and which is further influenced by the low frequency components of the detection signal. Accordingly, it is desirable to provide that the reproduced detection signal has a format with substantially no components thereof in a predetermined low frequency range.